The present invention relates to a semiconductor device having a fine structure, and in particular to a fine annular quantum wire structure which can be integrated, and a semiconductor device and a memory device using it.
Various semiconductor elements using silicon single crystals as a fundamental material are used widely and commonly. For enhancing the performance of these semiconductor elements, it is one of effective means to increase the transit speed (mobility) of electrons transiting in the material. The upper limit value of the mobility of electrons in single crystals is determined by physical property factors, and the upper limit value cannot be improved by the structure. In recent years, however, the fact that the mobility of electrons in a silicon crystal having strain is increased as compared with the original silicon crystal has been reported.
In a common method for fabricating such elements, a crystal slightly differing in lattice constant from the silicon crystal is prepared and thereon a silicon layer thinner than the critical film thickness is formed by using a thin film growing technique as means for providing the silicon crystal with a strain. To be concrete, a SiGe alloy crystal layer having a Ge composition of approximately 30% as an alloy crystal having lattice constants slightly greater than silicon is prepared, and thereon a silicon thin film layer having a thickness of 100 nm or less is formed. (In this case, lattice constants of the SiGe crystal are greater than those of the Si crystal by approximately 1.2%.) At this time, it is difficult to procure an inexpensive SiGe crystal substrate which is industrially mass-produced and which is excellent in quality. Usually, therefore, a silicon wafer is used as a substrate, and thereon SiGe having a sufficient thickness (thicker than the critical film thickness) is formed to obtain a crystal which is slightly greater in lattice constant than Si.
The Si layer thus formed is subjected to tensile strain. As a result, not only the mobility of electrons is improved, but also there is obtained such an advantage that the potential position of the conduction band is moved to a lower position as compared with the SiGe layer by the effect of the tensile strain and consequently electrons can be stored more easily. There is an example in which n-type MOSFETs and MODFETs were fabricated by way of trial by using such characteristics. On the other hand, for improving the mobility of holes, it is more advantageous to use a SiGe layer having compressive strain as compared with the Si layer having tensile strain fabricated as described above. In other words, a SiGe layer with no strain relaxation caused becomes necessary in this case.
For thus forming a p-type MOSFET (or MODFET) with a hole mobility increased by using the strain of a crystal, there is required a thin film layer structure (modulated dope structure) different from that in the case where the n-type MOSFET (MODFET) is formed. In other words, for forming a CMOS incorporating both an n-type FET and a p-type FET, layered structures suited for both the n-type FET and the p-type FET need to be formed separately. Therefore, conventional hetero devices had a big problem in integration.
In a quantum confined structure, electrons or holes are confined in an extremely small area in a semiconductor material (to be concrete, an area having dimensions shorter than the wavelength of electrons). Such a quantum confined structure has unique characteristics which cannot be obtained in conventional semiconductors. Therefore, its fabrication results by way of trial using various methods have been reported. Above all, a quantum well structure can be easily formed. In the case of the quantum well structure, a layer is formed from two kinds of semiconductors having different band gaps and quantum confinement is conducted in only one direction. A large number of examples of application thereof have been reported.
As for a quantum wire structure with quantum confinement conducted in two directions and a quantum box structure with quantum confinement conducted in all of three-dimensional directions, it is difficult to form them and reports of them are few.